The present invention relates generally to pulp screens for use in screening or filtering stock used in making paper and other fiber related products, and more particularly to slotted screen baskets for use in a pulp screening apparatus.
Perforate screens are often used to filter liquid. For example, screens are used to filter particulates entrained in water, such as debris in wastewater or fiber in a stock slurry. For example, screens are used to remove debris from a stock slurry including other fiber that is undesirable for use in making fiber products. Screens are also used to remove debris from a slurry containing aggregates and/or powder. Screens can be used for other filtering or screening application.
Screens are also used to classify matter in a liquid. For example, screens are used to filter out matter having a certain size and greater (or smaller) from a flow of liquid. Similarly, screens filter out particles of a certain size or size range while permitting other particles having a certain size or size range to remain in the liquid. Classifying applications include classifying fiber in a slurry for papermaking applications, classifying powder in a slurry, classifying aggregates in a slurry, and classifying other materials entrained in a flowing liquid.
Each screen is of perforate construction having a plurality of pairs of perforations or holes that each permit liquid and matter to pass through. The size and shape of each hole is selected for the particular filtering or classifying application and determines what matter is removed from the liquid and what matter is retained.
One type of screen is generally cylindrical and has a plurality of generally cylindrical perforate metal sidewalls that are disposed generally along a common central axis and which both are affixed to metal support rings. For example, where the screen has two such cylindrical sidewalls, there is a single intermediate support ring between the two sidewalls and an end ring at each end. The rings are connected by spaced apart rods that are tightened to keep the rings and cylindrical sidewalls captive in compression.
Each metal sidewall is made of a rectangular panel that is first machined to form screening holes therein, rolled into a cylinder, and then welded to retain its cylindrical shape. In the pulp industry, each sidewall is referred to as a screen section and the screening holes are shaped like slots.
Before assembly, the rings and the sidewalls must be machined to form suitable weld joints. In assembling the screen, the sidewalls and rings are received in an expensive and complicated welding fixture. Prior to welding, each of the rings are heated to shrink fit them around each screen sidewall. During welding, expensive welding equipment is used to weld each support ring to at least one of the metal sidewalls. The intermediate support ring is welded to both sidewalls.
After welding, post-weld grinding and finishing must be performed to clean each weld, remove weld splatter, and remove weld discoloration. In an attempt to relieve thermal stresses introduced by all the welding that is performed, the assembled and welded screen is heat treated.
Welding is expensive, time consuming, costly and produces a screen with limitations. The fixtures and equipment used to weld are expensive, complicated and time consuming to set up, and require a great deal of labor. A screen produced by welding, even by laser or electron beam welding, has stresses present, even after stress relieving, that can limit the useful life of the screen. As the cycles that a screen is inevitably subjected to during operation accumulate, premature failure can occur because the screen has not been adequately stress relieved. Even if adequately stress relieved, cracks can still form in a weld or in the heat affected zone of a weld. Screen performance can also be adversely affected by welding-induced thermal distortion that is not eliminated by stress relieving.
Welding also can reduce the total area available for screening. For example, where the screen is held together by conventional welds, the welds that join each support ring to a sidewall limit the space available for screening holes or slots by at least three or four millimeters adjacent each ring about the entire periphery of the screen.
Thus, the need exists for a screen which lacks at least some of these disadvantages.
Pursuant to one aspect of the invention, a screening assembly is provided for a screening apparatus that includes at least one perforate sidewall that is joined by a bonding material to a support ring. In one embodiment, the perforate sidewall is a slotted screen section and is joined by bonding material, preferably brazing material, to the support ring. The resultant screening assembly preferably is of monolithic or unitary construction that is strong, tough, and resilient.
The screen section is of metallic construction. In one preferred embodiment, the screen section is made of a stainless steel that preferably is of austenitic construction. The screen section preferably is shaped into a cylinder or hoop and has a plurality of pairs of spaced apart and elongate slots that each includes a slit that extends completely through the cross section of the screen section. In one preferred slot construction, the slot is formed by a contour cut on one side and a back groove on the other side that is aligned with the contour cut producing intersects of the slot to produce the slit. In another preferred slot construction, the back groove has a material relief that removes residual material between the contour cut and back groove at each end of the slit to thereby extend the length of the slit. Such a screen section is well suited for screening pulp fiber in a stock slurry.
In one preferred screening assembly, the screening assembly is a screen basket that is comprised of at least one perforate screen section that carries a pair of spaced apart support rings. Each support ring bounds a periphery of the screen section that preferably is the outer periphery. In another preferred screen basket, the basket is of modular construction and includes at least two screen sections, a pair of end support rings, and at least one intermediate support ring. An intermediate support ring preferably is disposed in between each pair of adjacent screen sections.
There is a joint between each support ring and the screen section where the ring and screen section weldlessly bond. Where the screen basket has end support rings, there can be as little as a single joint between each end ring and the screen section to which it is bonded. Where the screen basket has one or more intermediate support rings, there are at least two joints per intermediate support ring. For example, there is at least one joint formed between the intermediate support ring and each screen section to which it is bonded.
Each joint preferably is defined by at least one faying surface in the ring and at least one faying surface in the screen section forming a set of faying surfaces that preferably abut or are adjacent. The bonding material bonds the faying surfaces to weldlessly attach the ring to the screen section. Preferably, there is at least one set of faying surfaces that are parallel to a longitudinal central axis of the screen basket. To provide sufficient space between each set of faying surfaces to accommodate bonding material, there is a gap or tolerance between each set of faying surfaces. In one preferred embodiment, the gap or tolerance is no greater than 0.008 inches. In one preferred embodiment, each joint can have two sets of spaced apart faying surfaces that are each substantially parallel to the longitudinal central axis of the screen basket. One preferred joint configuration is a tongue and groove joint where there is a tongue formed in either a screen section or the ring and a groove formed in the other component.
Where the bonding material is a brazing material, the brazing material is a brazing alloy that preferably in paste form. One preferred brazing alloy is a nickel base brazing alloy that includes one or more of the following constituents: Chromium, Boron, Silicon, and Iron. A preferred brazing alloy includes all of these constituents and Carbon and Cobalt and can be made in conformance with AMS 4777, AMS 4777F or American Welding Society (AWS) Braze filler metal specification Bni-2. The brazing alloy can also include one or more of the following constituents: Aluminum, Phosphorous, Sulfur, Titanium, and Zirconium.
Pursuant to another aspect of the invention, a method of making a screen basket comprises weldlessly bonding support rings to a screen section. In preparation to bond each ring to a screen section, at least the screening holes or slots are covered, such as by a masking material. A layer of bonding material is disposed in each joint between each ring and screen section.
In one preferred implementation of the method, the bonding material is brazing alloy that is placed in each joint, preferably along the entire length of the joint. The brazing alloy is heated to a temperature above which it becomes flowable or a liquid. Preferably, the brazing alloy is heated to a temperature above its liquidus temperature to braze each ring to a screen section.
To help facilitate wetting of each faying surface of each joint with molten brazing alloy, a vacuum can be applied. In one preferred implementation of the method, the vacuum is at least 4xc3x9710xe2x88x924 torr or better. In another preferred implementation of the method, the vacuum is at least 1xc3x9710xe2x88x924 torr.
To help prevent carbide formation, the assembled and brazed screen basket is quenched. Quenching, preferably nitrogen quenching, is performed to lower the temperature to a temperature below the solidus temperature of the brazing alloy. In one preferred implementation, the temperature of the brazed screen basket is lowered after brazing to a temperature of about 150xc2x0 F. (66xc2x0 C.) in less than about twenty minutes.
In one preferred implementation of the method, during brazing, at least the screen section is heated to a temperature at or above which stress relieving and/or annealing occurs. The temperature is held at or above the annealing and/or stress relieving temperature at least until annealing occurs. Oxidation on the outside of the screen basket is removed during annealing.
In another preferred implementation of the method, at least the screen section is heated after annealing and/or stress relieving to a temperature at or above which boron diffusion occurs. The temperature is held at or above the boron diffusion temperature at least until boron diffusion occurs.
An oven that preferably is a vacuum furnace preferably is used in brazing. The vacuum preferably is applied during annealing and boron diffusion. During brazing, the screen section and brazing alloy is heated to a temperature of at least 1650xc2x0 F. (899xc2x0 C.). Where the screen section is made of an austenitic stainless steel, at least the screen section is heated for at least ten minutes to a temperature of at least 1850xc2x0 F. (1010xc2x0 C.) and preferably no greater than 1925xc2x0 F. (1052xc2x0 C.) to anneal the screen section. Where boron diffusion is performed, at least the screen section is heated for at least one hour and as much as two hours to a temperature of at least 1700xc2x0 F. (927xc2x0 C.) and preferably no greater than 1900xc2x0 F. (1038xc2x0 C.). Quenching is performed at the end to reduce the temperature to about 150xc2x0 F. (66xc2x0 C.) in about twenty minutes or less.
After brazing is completed, any masking material is removed. In one preferred implementation of the method, the masking material is removed by sandblasting, which also deburrs slots machined into each screen section. After sandblasting, the screen basket can be electropolished to smooth sharp edges. After electropolishing, the screen basket can be chrome plate.
Objects, features, and advantages of the present invention include one or more of the following: a screening assembly and method of making the same that requires no fixturing to keep components in precise alignment during bonding; eliminates the need for any post-weld heat grinding and finishing; eliminates the need to shrink fit support rings; maintains dimensional stability and eliminates virtually all dimensional distortion associated with welding; produces a finished screening assembly that is bright and shiny without discoloration and which accepts chrome plating well; requires no further machining after bonding is complete; enables filter slots to extend to each adjacent support ring thereby increasing filtering area; increases screening assembly strength; eliminates screen section waviness found in welded screening assemblies; makes rotor clearance more uniformly parallel in the axial direction along the entire axial length of the rotor which increases screening assembly performance; produces a screening assembly that is fully solution annealed, which improves corrosion resistance; stress relieves all weld joints in all screen sections and rolled support rings; enhances the quality of each such preexisting weld joint; makes the hardness of each such preexisting weld joint more uniform throughout the cross section of the joint; improves the corrosion resistance of each preexisting weld joint; increases the surface area available for screening; produces a bonded screening assembly that is of simple, durable, tough, monolithic construction that lasts longer and which improves screen assembly performance; and a method that is simple, less labor intensive, cheaper, and which produces a screening assembly of higher quality construction that is dimensionally truer and which lasts longer at a higher level of performance.
Other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating at least one preferred embodiment of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.